Re: The ability of postgres to determine loss of files of the main fork

On Wed, Oct 1, 2025 at 1:46 PM Aleksander Alekseev
<aleksander(at)tigerdata(dot)com> wrote:
>
> Hi Jakub,
>
> > IMHO all files should be opened at least on startup to check integrity,
>
> That might be a lot of files to open.

I was afraid of that, but let's say modern high-end is 200TB big DB,
that's like 200*1024 1GB files, but I'm getting such time(1) timings
for 204k files on ext4:

$ time ./createfiles # real 0m2.157s, it's
open(O_CREAT)+close()
$ time ls -l many_files_dir/ > /dev/null # real 0m0.734s
$ time ./openfiles # real 0m0.297s , for
already existing ones (hot)
$ time ./openfiles # real 0m1.456s , for
already existing ones (cold, echo 3 > drop_caches sysctl)

Not bad in my book as a one time activity. It could pose a problem
potentially with some high latency open() calls, maybe NFS or
something remote I guess.

> Even if you can open a file it doesn't mean it's not empty

Correct, I haven't investigated that rabbithole...

> or is not corrupted.

I think checksums guard users well in this case as they would get
notified that stuff is wonky (much better than wrong result/silent
data loss)

-J.